I bought 3 of these today for $38 each.
Note that they have a learning mode to discover the P, I and D values to be used in the system.
Note that they also have a built in relay that will directly control my gas valves. (24VAC @ 0.5A)
I love open source more than the next guy... in both Linux and Arduino.
However, if I was going to do this sort of thing, I would use a device like this
Fusion Brain Version 6
and read the RTD voltages into a laptop, do all the processing, display, logging, etc there and then use the out pins to turn relays on and off.
15 analog inputs and 31 digital outputs should be enough to run a brewstand, right ? Draft up a controller GUI using PyQT(4) and away you go.
I'm kind of kicking myself for buying those controllers today. I want to get brewing in February, so maybe I'll do the laptop thing next year.
BTW: I think that logging is the most important feature any brew system could have, so that after the fact you know what really happened with the mash.
Just my $0.02.